1. Field of the Invention
This invention relates to a device for purifying exhaust gas from an internal combustion engine.
The present application is based on Japanese Patent Application No. Hei. 10-61401, which is incorporated herein by reference.
2. Description of the Related Art
Nitrogen oxides, carbon monoxide and hydrocarbons are contained in exhaust gas discharged from an internal combustion engine mounted on a vehicle such as an automobile, and after these are purified through the oxidation-reduction effected by a three-way catalyst, the exhaust gas is discharged to the atmosphere. Precious metal, such as platinum (Pt), rhodium (Rh) and palladium (Pd), is used in this three-way catalyst, and performs an efficient exhaust gas purification effect in a high-temperature, activated condition of not less than about 350.degree. C. Namely, in a cold condition of the engine as when starting the engine in a low-temperature condition, the exhaust gas purification effect, achieved by the catalytic converter, is extremely aggravated. Therefore, the temperature of the catalytic converter needs to be rapidly raised to a predetermined temperature so that the converter can be brought into a high-temperature, activated condition. Therefore, there is known an electrically heated-type heater (called "electrically-heated catalyst" (EHC) unit) in which the electrically heated-type heater is provided upstream of a catalyst converter unit so as to heat exhaust gas.
FIG. 1 shows one example of an exhaust gas purification device incorporating an electrically heated-type heater. In the sequence from a downstream to an upstream side with respect to the direction of flow of exhaust gas, an exhaust pipe 101, main catalytic converter units 102 and an auxiliary catalytic converter unit 103 (these converter units 102 and 103 jointly constitute an exhaust gas purification catalytic converter) are arranged in this order. Further, an EHC unit 104, containing the electrically heated-type heater, is provided immediately upstream of the auxiliary catalytic converter unit 103. In the catalytic converter of this construction, when an internal combustion engine is in a cold condition as at the time immediately after starting the engine, a switch SW is operated to apply a battery voltage VB to the EHC unit 104 to heat the same. As a result, the electrically heated-type heater of the EHC unit 104 is heated to promote the oxidation of unburned components of hydrocarbons in the exhaust gas, and this oxidation heat cooperates with the heat, generated by the electrically heated-type heater, to rapidly heat this electrically heated-type heater. As a result, the exhaust gas is heated, and also the auxiliary catalytic converter unit 103 of the next stage is heated, and further the main catalytic converter units 102 are heated, so that the whole of the catalytic converter is rapidly activated.
For example, Japanese Patent Publication No. Hei. 8-218856 discloses an electrically-heated catalyst (catalytic converter). A pair of electrodes are provided on a catalyst support comprising a honeycomb structural body having exhaust flow passages, and a plurality of slits are formed in the catalyst support so that the catalyst support, when electrically heated, can be heated uniformly over an entire area thereof, and in this construction, thermal stresses, developing at ends of the slits, are relieved.
FIG. 2 shows one specific construction of the EHC unit 104. In this case, an electrically heated-type heater comprises a flat, cross-sectionally circular honeycomb structural body 1 having exhaust flow passages, and this heater itself is heated by resistance heat upon application of an electric current. The honeycomb structural body 1 may be provided in the form of a monolith catalyst, in which case the three-way catalyst is dispersed and deposited on the surface of the catalyst support (honeycomb structural body). This honeycomb structural body 1 is received within a housing 3 of metal, and is retained there in an insulated manner by retaining means (not shown). A pair of electrodes 2 and 2' extend through a peripheral wall of the housing 3, and supply electric power to the honeycomb structural body 1 from the exterior. If the electrodes 2 and 2' are connected respectively to diametrically-opposite portions of the honeycomb structural body 1 of a circular cross-section, a large proportion of the electric current flows along the shortest path interconnecting the electrodes 2 and 2. Namely, the value of the current varies from one portion to another of the honeycomb structural body 1, so that the temperature distribution due to the heating becomes non-uniform. In order to overcome this problem, the honeycomb structural body 1 has a plurality of slits 4 parallel to one another. With this construction, the electric current flows between the electrodes 2 and 2' along a zigzag path formed as a result of the formation of the slits 4, and therefore the concentration of the electric current is avoided, and the honeycomb structural body 1 is heated uniformly over the entire area thereof.
As is well known, exhaust gas from an internal combustion engine contains pyrolytic high molecular substances and free carbons due to engine oil, and abrasion powder resulting from a cylinder wall and a piston ring. Larger ones among these foreign matters in the exhaust gas may physically damage the EHC unit 104 upon impingement thereon. Even smaller or fine foreign matters adhere to and deposit on the honeycomb structural body 1 and electrodes 2 and 2' of the EHC unit 104 which are disposed in the exhaust passage, and are exposed directly to a stream of exhaust gas. The foreign matters, deposited on the honeycomb structural body 1, cause the clogging of the exhaust flow passages, and adversely affect the exhaust gas purification properties. Particularly when electrically-conductive foreign matters adhere to the EHC unit 104, firstly, the slits 4, formed in the honeycomb structural body 1, are short-circuited, so that the proper zigzag current path cannot be maintained, thereby preventing the honeycomb structural body 1 from being heated uniformly over the entire area thereof. Secondly, the ability of insulation between the electrodes 2 and 2' and the metal housing 3, as well as the ability of insulation between the honeycomb structural body 1 and the metal housing 3, is lowered, so that the predetermined voltage cannot be applied to the honeycomb structural body 1. These all adversely affect the exhaust gas purification effect achieved by the catalytic converter.